PERSISTENT DUCTUS ARTERIOSUS IN DOGS AND CATS
Patent or patent ductus arteriosus (PDA) is a very common congenital cardiac anomaly in dogs and also exists, but much less common, in cats. This is the first of a series of articles in which we are going to answer some common questions and doubts about the most important aspects of this heart defect.
What is the difference between ductus arteriosus and patent ductus arteriosus?
The ductus arteriosus is a fetal blood vessel that connects the pulmonary artery and the aorta once the heart is formed . During the middle and advanced phase of gestation the heart has a double parallel circulation communicated. Gas exchange is carried out by the maternal lung, with oxygenated blood being transported through the placenta and umbilical veins to the circulatory system of the fetus.
In this context, pulmonary circulation is greatly reduced due to pulmonary arterial vasoconstriction, and the consequent high pressures in the pulmonary artery, caused mainly by the absence of oxygen in the alveoli.
Most of the blood that reaches the pulmonary artery flows directly to the aorta, which currently has lower pressures than the pulmonary artery, through the ductus arteriosus as we explain later with the images of the heart.
The ductus arteriosus is therefore essential for the correct functioning of the circulatory system during the fetal phase. Immediately at birth, however, a series of changes occur in the puppy that will lead to the establishment of the characteristic circulation of mammals: the double parallel circulation in series (without communication).
These changes include: the entry of oxygen into the lungs with consequent pulmonary arterial vasodilation and a marked decrease in pulmonary arterial pressure; the closure of the foramen ovale; closure of the ductus venosus in the liver; and the closure of the ductus arteriosus, which should be completely closed by seven days of life.
If the ductus remains abnormally open, with flow between the aorta and the pulmonary artery, the patient is considered to have a patent ductus arteriosus (PDA) .
Although the terms patent and persistent are used synonymously, persistent is more often used in human medicine to refer to the PDA in the premature neonate and patent to that duct with flow in the non-premature baby.
Appearance of the heart, aorta, pulmonary artery and ductus arteriosus in three different physiological/pathophysiological situations
Red blood represents arterial blood (with high oxygen content). Blue blood represents venous blood (with low oxygen content). The areas with mixed color represent blood with a mixture of arterial and venous blood. In figure A , in the fetus, red blood is the most oxygenated but does not have the arterial levels as after birth (saturations of 70% are common). The most oxygenated blood reaches the right atrium through the caudal vena cava (80% saturation; represented in the upper part of the right atrium) and passes, after slightly mixing with that coming from the cranial vena cava, through the foramen ovale, to the left ventricle.
A) In this case the alveoli are physiologically collapsed without the presence of oxygen and there is physiological pulmonary arterial/arteriolar vasoconstriction . The pressures of the pulmonary artery are high and greater than those of the aorta, so there is a flow through the ductus arteriosus with a direction, normal for this stage, from right to left.
B) In this case the flow of the patent ductus arteriosus is from left to right since the pressures in the pulmonary artery are, as in all normal puppies, much lower than those in the aorta in both systole and diastole . This continuous flow creates volume overload and enlargement of the pulmonary artery, pulmonary veins, left atrium, left ventricle, and aorta. In this case there is no passage of venous blood to the general arterial circulation (there is no hypoxemia due to the shunt ).
C) In this case, the puppy has abnormal and higher pressures in the pulmonary artery (pulmonary hypertension) than in the aorta, which leads to a flow from right to left through the shunt and a mixing of venous and arterial blood in systemic arterial circulation . This is more pronounced in the lower part of the body (which can cause differential cyanosis) since the brachiocephalic trunk and left subclavian artery, which nourish the head, neck and forelimbs, leave the aorta in an anatomical location anterior to the PDA. The heart, due to this pulmonary hypertension, has an abnormal remodeling with dilation of the pulmonary artery, concentric or eccentric hypertrophy of the right ventricle and often also an increase in the size of the right atrium.
In the following posts we tell you more about CAP: clinical findings, diagnosis, treatment and prognosis.